The present invention relates to an ignition apparatus of low-voltage wiring type for an engine using gasoline as a fuel.
In an automotive gasoline engine, it is widely utilized to supply lean mixture into the engine and combust it completely in order to meet the restrictions against environmental pollution. In the engine using the lean mixture, therefore, an accurate timing advance control is required over a wide range of ignition timing. In order to meet this requirement, a low-voltage wiring system has been put into practical use. In the low voltage wiring system, a distributor is eliminated and the ignition apparatus is arranged in each cylinder. Another advantage of the low-voltage wiring system is that the absence of a high-voltage wiring leads to a reduced trouble of the electrical system, and the wiring is simplified.
For an independent ignition apparatus to be arranged in each cylinder of a multicylinder engine, the ignition apparatus is required to be compact and slim. Under the circumstances, however, a conventional ignition apparatus which was combined with the distributor was used by reducing the size. Therefore, the efficiency was low and the reliability was not high.
A first prior art ignition apparatus will be explained with reference to a circuit configuration of the first prior art ignition apparatus shown in FIG. 10.
A primary winding 31 of a transformer (ignition coil) 3 is connected to a battery 1 through a switching element 2. An end of the secondary winding 32 of the transformer 3 is connected to the negative electrode of the battery 1, and the other end thereof is connected to a spark plug 33. FIG. 11A shows the current flowing in the switching element 2, and FIG. 11B a current in the secondary winding 32.
The switching element 2 is turned on/off by a control signal applied thereto from a controller not shown. Upon turning on of the switching element 2, a current flows through the battery 1, the primary winding 31 and the switching element 2 so that an electromagnetic energy is stored in the transformer 3. An on-period of the switching element is designated as T.sub.on. At the time when the switching element 2 is turned off, the electromagnetic energy stored in the transformer 3 is represented by (Cs.multidot.Vs.sup.2)/2, where Cs is a distributed capacity of the secondary winding 32 and Vs is a secondary voltage. And when the switching element 2 turns off, the stored energy is transferred to the secondary side. As a result, the secondary voltage Vs rises to such an extent that plug gap 34 of a spark plug 34 breaks down and a discharge current flows. A transistor or a FET is generally used as the switching element 2.
A capacitor discharge ignitor (CDI) disclosed in JP-A-60-252168 is shown in FIG. 12 as a second prior art. FIG. 12 shows a circuit configuration of the CDI. A battery 1 and a spark plug 33 are substantially identical to those shown in FIG. 10. A DC-DC converter 4 in series with a capacitor 5 is connected between the positive terminal of the battery 1 and the primary winding 31 of the transformer 3. A switching element 2A is inserted between the junction point between the DC-DC converter 4 and the capacitor 5 and the negative terminal of the battery 1. The switching element 2A requires a high allowable pulse current value, and therefore generally is composed of a thyristor. FIG. 13A shows a current flowing in the switching element 2A, and FIG. 13B a discharge current flowing in the secondary winding 32.
In FIG. 12, the voltage across the battery 1 is converted to a high DC voltage (e.g. 400 v) by the DC-DC converter 4 and charges the capacitor 5. A pulse signal responding to an ignition timing is supplied to the gate of the switching element 2A from a controller not shown, and the switching element 2A turns on. A charge stored in the capacitor 5 is discharged through the switching element 2A and the primary winding 31 of the transformer 3. Thus, a high voltage is generated across the secondary winding 32, and a discharge current of FIG. 13B flows. The discharge current from the capacitor 5 assumes a resonance waveform determined by an equivalent inductance as viewed from the primary side of the transformer 3 and the capacitance of the capacitor 5. In order to turn off the thyristor positively in preparation for the next firing, it is a general practice to turn on the thyristor only during the positive half cycle and turn it off during the next negative half cycle. In the first conventional ignition apparatus, the transformer 3 has dual functions of storing the electromagnetic energy and boosting the voltage. As regards the energy storage, however, an inductance device have a low volume ratio as described below. The number of turns of the primary winding 31 is determined by the inductance required for the electromagnetic energy storage. Further, the requirement for a large step-up ratio greatly increases the number of turns of the secondary winding 32. As a result, the distributed inductance and the distributed capacitance are increased, thereby adversely affecting the energy transfer efficiency of the transformer.
Further, it is necessary to turn on the switching element 2 before the desired ignition timing. This timing is determined based on the information on the previous cycle. It is therefore difficult to control the turn-on timing accurately following the sudden change of the engine speed.
In the CDI system of the second prior art, the energy storage element is the capacitor 5. The capacitor 5 is smaller than the transformer 3 for the same energy storage, and therefore the energy storage element can be reduced in size. The transformer 3 is not required to store energy, and can be greatly reduced in size, because the magnetic saturation due to the exciting current is the sole matter of consideration. For example, the number of turns of the primary winding of the transformer in the second prior art is about one third of that in the first prior art. Thus the energy transfer efficiency is high. In view of the fact that the thyristor is used for the switching element 2A, however, the discharge time has to be shortened in order to prevent a firing error. Furthermore, since the switching element 2A is connected across the output terminal of the DC-DC converter 4 and the negative terminal of the battery 1, the battery 1 is shortcircuited by the on-state of the switching element 2A. Therefore, the on-period of the switching element 2A can not be extended. The low ignition accuracy, therefore, has been the problem for the lean mixture requiring a long discharge time, 0.5 milliseconds for example.